Writing for The Huffington Post UK, Watson dismissed the idea Johnson's wish to leave the European Union is a "sincerely held view" and is instead a "political calculation" to "increase his chances of becoming Prime Minister".

Johnson reportedly wrote two columns for The Telegraph when announcing his intention to leave - one for Leave and one for Remain.

Watson's attack is notable for Labour ramping up the rhetoric against him against the likelihood of Johnson being the Tory leader at the next 2020 election.

Jonathan Brady/PA Wire

Labour party deputy leader Tom Watson

In one striking extract, he says:

"This referendum has been hijacked by one man who never fails to place political ambition before principle.

"Boris Johnson has decided to use the EU vote to further his own career and force his way into No 10 - regardless of the cost to the country and seemingly his own party.

"It is cynical. It is short-sighted. It is selfish. And I believe the British electorate will see through it."

Watson details how Johnson two years ago wrote: "Leaving the EU will be globally interpreted as a narrow, xenophobic, backward-looking thing to do."

"Now he is the cheerleader for the Leave campaign," Watson adds.

Even before his announcement at an impromptu press conference, some made clear they thought he was not a true believer - including Tory MP Nicholas Soames, the grandson of Winston Churchill.

Whatever my great friend Boris decides to do I know that he is NOT an outer .